1. Field of the Invention
The present invention relates to an electromagnetic motor of the finite rotational type for accurately rotating a light reflector or the like between predetermined angular positions. In particular, the present invention pertains to an electromagnetic motor of the finite rotational type which is provided with an improved mechanism for returning the rotor thereof to its initial angular position.
2. Prior Art Description
The electromagnetic motor of the finite rotational type is widely utilized as a source of rotating motion for rotational members which must be rotated to a prescribed position accurately with high response. For example, in the field of optical scanning, this kind of motor is utilized to rotate a light reflector for deflecting or scanning a light beam. This type of electromagnetic motor is disclosed, for example, in Japanese Patent Publication No. SHO 57-56304. Typically, an electromagnetic motor of the finite rotational type has a stator enclosing a rotor made of ferromagnetic material, the stator being provided with a magnetic flux generating mechanism which generates a magnetic flux for rotating the rotor within a predetermined range of angular positions. The magnetic flux generating mechanism has a plurality of magnetic pole portions, around which respective coil windings are wound. The electromagnetic motor of the finite rotational type is also provided with a sensing means for sensing the rotational angular position of the rotor. The sensing means is typically of the capacitance position transducing type.
The electromagnetic motor of this type is also provided with a mechanism which exerts a force for returning the rotor to its initial angular position. Since the rotor does not operate by excitation when it falls into the position at which the poles thereof are aligned with those of the stator, it must be returned to a predetermined angular position at which the poles thereof are angularly offset from those of the stator by a predetermined amount. The mechanism for returning the rotor to its initial angular position is provided for this purpose. In the above-mentioned Patent Publication, a torsion bar is accommodated along the rotational axis of the rotor for exerting a force to return the rotor to its initial angular position when de-excited. However, the rotor-returning mechanisms of the prior art have defects. Since they consist of many elements which have to be assembled, they are complicated in structure and prone misalignment. When the rotor-returning mechanism is misaligned, the rotor does not return to its initial position accurately. In addition, the rotor may be deflected radially as it rotates.
The rotor of an electromagnetic motor of this type is generally rotatably supported at both ends by bearing means. The rotor is desirably supported by the bearing means such that it does not deflect radially when rotating. However, due to manufacturing inaccuracy, misalignment or the like of the bearing means, the rotor is inevitably supported in a condition slightly deflectable in the radial direction. The rotor therefore deflects radially when rotating. The radial deflection of the rotor degrades the accuracy of the sensing mechanism for sensing the rotational angular position of the rotor.